🧠 Equine Influenza (Horse Flu) Vet Guide 2025 by Dr Duncan Houston

Equine influenza (EI), commonly known as horse flu, is one of the most rapidly spreading respiratory viral diseases in equines. In this comprehensive guide, I—Dr Duncan Houston BVSc—explain the virus, how it spreads, clinical signs, diagnostics, treatment, recovery protocols, vaccination strategies, biosecurity measures, and outbreak prevention. 🐎✨

1. What Is Equine Influenza?

EI is caused by influenza A viruses, primarily the H3N8 subtype (H7N7 is now largely extinct) :contentReference[oaicite:3]{index=3}. It attacks respiratory epithelial cells, leading to inflammation and impaired mucociliary clearance :contentReference[oaicite:4]{index=4}. Worldwide, it occurs sporadically or in outbreaks, especially where horses mix—events, training barns, sales :contentReference[oaicite:5]{index=5}.

2. How It Spreads

The virus transmits primarily through aerosol droplets from coughing or sneezing. Contaminated objects—tack, grooming gear, buckets—can carry it, allowing indirect spread :contentReference[oaicite:6]{index=6}. Infected horses start shedding virus 1–3 days post-exposure, often before clinical signs appear, and can continue shedding for up to 7–10 days :contentReference[oaicite:7]{index=7}. The virus can remain viable on surfaces for up to 48 hours :contentReference[oaicite:8]{index=8}.

3. Clinical Signs & Course

Equine influenza typically begins abruptly, with symptoms appearing 1–5 days after exposure :contentReference[oaicite:9]{index=9}.

• Fever: up to 106 °F (41 °C), usually lasting 2–3 days :contentReference[oaicite:10]{index=10}.
• Nasal discharge: watery clear discharge, may progress to mucopurulent if secondary infection occurs :contentReference[oaicite:11]{index=11}.
• Dry, harsh cough: persistent and productive of irritation, often lasting weeks :contentReference[oaicite:12]{index=12}.
• Lethargy & anorexia: affected horses show reduced appetite and willingness to move :contentReference[oaicite:13]{index=13}.
• Swollen lymph nodes: especially submandibular :contentReference[oaicite:14]{index=14}.

Usually not fatal in healthy adult horses, but complications like pneumonia, pleuritis, or poor performance can arise, especially in debilitated or young horses :contentReference[oaicite:15]{index=15}.

4. Diagnosis

Definitive diagnosis requires lab confirmation:

• Viral detection: via PCR or viral culture on nasal swabs collected early in illness :contentReference[oaicite:16]{index=16}.
• Serology: paired titers (acute vs convalescent samples) indicate recent infection :contentReference[oaicite:17]{index=17}.
• Clinical diagnosis: based on signs, fever, outbreak context, but should be lab-supported :contentReference[oaicite:18]{index=18}.

In vaccinated horses, viral shedding duration is shorter, and signs are typically milder :contentReference[oaicite:19]{index=19}. Vaccination status complicates testing; timing is critical.

5. Treatment & Care

No direct antiviral treatment is approved for EI—management is supportive and symptomatic :contentReference[oaicite:20]{index=20}.

5.1 Rest & Isolation

Rest is essential: at least 1 week off for every day of fever, with a minimum of 3 weeks total rest to allow respiratory epithelium to regenerate :contentReference[oaicite:21]{index=21}. Isolation helps prevent spread.

5.2 Supportive Measures

• Clean, well‑ventilated, dust‑free stabling.
• Encourage hydration and palatable feed; offer soaked hay.
• NSAIDs (flunixin or phenylbutazone) for fever and discomfort :contentReference[oaicite:22]{index=22}.

5.3 Antibiotics & Additional Care

Antibiotics are indicated only if bacterial pneumonia or prolonged fever (>3 days) occurs :contentReference[oaicite:23]{index=23}. Avoid strenuous exercise during recovery—risk of severe pulmonary damage.

6. Prognosis & Complications

Prognosis is good for otherwise healthy, vaccinated horses; most recover fully in 2–3 weeks, though coughing may persist. Unvaccinated, young, or immune-compromised horses risk pneumonia, prolonged recovery, or poor performance :contentReference[oaicite:24]{index=24}.

7. Vaccination Guidelines

Vaccination is the cornerstone of prevention. The AAEP and OIE recommend primary vaccination followed by boosters at:

• Foals: start at ~6 months, booster 3–6 weeks later, then at 10–12 months :contentReference[oaicite:25]{index=25}.
• Adults: annual booster; for competition horses, boosters every 6 months :contentReference[oaicite:26]{index=26}.
• Vaccines should include current H3N8 strains aligned with OIE recommendations :contentReference[oaicite:27]{index=27}.

Vaccination reduces clinical severity and viral shedding, even if it doesn’t always prevent infection :contentReference[oaicite:28]{index=28}.

8. Biosecurity & Outbreak Management

• Immediately isolate suspected cases; limit farm access.
• Clean and disinfect shared equipment; use PPE as needed :contentReference[oaicite:29]{index=29}.
• Quarantine new arrivals for 14 days, ensuring up-to-date vaccinations :contentReference[oaicite:30]{index=30}.
• Restrict movement off-farm during outbreaks; report to regulatory bodies if required.
• Monitor temperatures of all horses during outbreaks; treat fevers early.

9. Outbreak Response Strategy

1. Begin fever monitoring twice daily during high-risk seasons.
2. Isolate febrile/coughing horses immediately.
3. Contact your veterinarian for testing within 48 hours of symptom onset.
4. Initiate rest and supportive care pending results.
5. Reassess vaccination status; booster affected and at-risk horses.
6. Maintain isolation until 14 days post-last fever; resume activity gradually.
7. Review biosecurity protocols—improve quarantine, hygiene, and traffic control.

10. Economic & Industry Impact

EI outbreaks can halt events, impact racing schedules, and impose quarantine restrictions—leading to significant losses. For example, the 2007 Australian outbreak affected over 10,000 horses and cost an estimated $1 billion USD :contentReference[oaicite:31]{index=31}. Vaccination and biosecurity are key to maintaining industry stability.

11. Summary Table

🔚 Final Thoughts

Equine influenza is preventable and manageable when approached with diligence. Annual vaccination, careful biosecurity, early detection, and rest-based recovery allow most horses to return to full health. In the event of an outbreak or health concerns, don’t hesitate to contact our **Ask A Vet** team. Download the Ask A Vet app for 24/7 access to veterinary advice, outbreak support, vaccine reminders, and customized care plans. 🌟